ShortPill touchable region based on pill

When we're showing the short pill, we need to consume touched, otherwise
they will be routed to the Recents button. That said, the window is too
large, and we can't really block touches underneath, otherwise we'll
cause app issues.

Flag: com.android.systemui.enable_underlay
Test: expand/collapse ambient cue, with gesture or 3-button nav
Fixes: 415914274
Fixes: 415914083
Change-Id: I59d4ed8b0dd46c20df76624de34ca472901df482